Scaling up from a Numerator and Denominator to a Triple Ratio

Date: 07/08/2011 at 07:56:38
From: Jon
Subject: Ratio and Proportion
The ratio of mango trees to guava trees to apple trees in an orchard is
4:3:5. If there are 960 trees in the orchard, how many of each kind of
tree are there?
a. 320; 240; 400
b. 120; 240; 360
c. 240; 360; 480
d. 120; 280; 340
e. 20; 40; 60
Fractions typically consist of only two parts, but this problem presents
three unknowns in proportion with one another.
I've tried
number of mango trees = 960/4 = 240
number of guava trees = 960/3 = 320
number of apple trees = 960/5 = 192
But this answer is not one of the choices given.
Please help on how to solve this.